In recent years, for example, in order to perform high-speed and large-capacity communication, an optical wavelength division multiplexing (WDM) transmission method is known for transmitting an optical signal having a plurality of wavelengths in a multiplexed manner. In order to expand the transmission capacity, for example, a multiband system is known that uses a plurality of optical wavelength bands, such as a conventional band (C-band), a long band (L-band), and the like. C-band refers to, for example, an optical wavelength band of 1530 nm to 1565 nm. L-band refers to, for example, an optical wavelength band of 1565 nm to 1625 nm. In a WDM transmission method, a large power deviation (tilt) occurs at the short wavelength side and the long wavelength side of an optical fiber during transmission, for example, by having been affected by a wavelength dependent loss (WDL), stimulated Raman scattering (SRS), and the like. As a result, the quality of a signal on the reception side deteriorates. Accordingly, in order to reduce deterioration of the signal quality on the reception side, tilt compensation is requested.
However, in a multiband system, the optical wavelength band to be used is expanded, and thus the tilts that occur on the shortest wavelength side and the longest wavelength side of an optical signal transmitted becomes large compared with the case of a single-band system that uses only a single optical wavelength band. For example, a tilt due to a WDL depends on the characteristic of an optical fiber of the transmission path, and does not depend on the number of wavelengths that is changed during operation, and thus it is relatively easy to estimate a tilt quantity and to compensate for the tilt. In contrast, a tilt due to SRS depends on the number of wavelengths that is changed during operation, and thus tilt compensation has to be performed in accordance with the number of wavelengths.
Thus, in a tilt compensation technique for a multiband system, a method is known for monitoring the spectrum of a signal wavelength by a reception-side WDM apparatus, feeding back the monitor result to a transmission-side WDM apparatus, and compensating for a tilt by the transmission-side WDM apparatus based on the monitor result. In this method, it is possible to realize high-precision tilt compensation not only for a tilt that occurs on a transmission path between the reception-side WDM apparatus and the transmission-side WDM apparatus, but also in accordance with the number of wavelengths that is changed during operation. As the related art, for example, Japanese Laid-open Patent Publication Nos. 2002-368691, 2010-97987, 2006-295113, and 2001-53686, and the like are disclosed.
However, the transmission-side WDM apparatus obtains a monitor result from the reception-side WDM apparatus, and thus it takes a long time before tilt compensation is performed. For example, it is assumed that when the number of wavelengths is changed abruptly due to a disconnection of an optical fiber, a device failure, or the like, tilt compensation is not made in time, and thus a signal error sometimes occurs due to deterioration of the reception-side signal quality.
For a method of realizing high-speed tilt compensation, a method is provided for monitoring the input power of a WDM signal input to the transmission amplifier in a WDM apparatus and compensating for a tilt in the WDM signal based on the input power. However, in this method, although it is possible to make tilt compensation in a WDM signal of the same optical wavelength band, it is not possible to make compensation for SRS tilt that occurs among the WDM signals having different optical wavelength bands in a multiband system. Accordingly, in reality, a WDM apparatus is demanded that is capable of compensating for SRS tilt at high speed and with high precision in a multiband system.
In view of the above, it is desirable to provide an optical wavelength multiplexing transmission apparatus, or the like capable of compensating for an SRS tilt at high speed and with high precision in a multiband system.